The new Yahoo Mail was Yahoo's first major product release since Mayer joined the company. The reason the Yahoo board hired Mayer to be CEO is that she is widely considered to be a product genius. Did she deserve the reputation?

Yahoo Mail is a huge source of traffic and, therefore, revenue for Yahoo. But, because teenagers don't email much at all and adults check email on their phones, Yahoo Mail was losing ground. Could Mayer's update reverse the trend?

In the months prior to that December 11 launch, Mayer made it clear to her direct reports that she knew the stakes were high. She pushed the Yahoo Mail team incredibly hard. The team responded, working nights and weekends for months.

It was a huge change of pace for Yahoo. The last major update to Yahoo Mail took the company 18 months to ship. With Mayer cracking the whip over Yahoo product leader Shashi Seth, the newest Yahoo Mail only took four months to build.

This didn't happen without some stress. Near the very end of the product development process – "the eleventh hour," says a source – Mayer decided that the new Yahoo Mail's colors were all wrong. It needed to be purple and yellow, she said, to match Yahoo's iconic colors.

Designers threatened to quit and there was a lot of grumbling, but the job got done – in remarkable time.

But now there are signs that Mayer pushed to get the product done too fast.

At the end of May, British Internet service-provider BT announced it was switching its 6 million users off of Yahoo Mail. A BT spokesperson said that users had been complaining about bugs in the product for months on BT's message boards, and that Yahoo had been unable to fix the issues.

That was bad news, but now there's worse.

We conducted a survey of 1,200 readers, and the results are brutal for Mayer and Yahoo.

That 57% statistic and the negative feedback on the message boards has to drive her nuts.

Here are some of these users' specific complaints about Yahoo Mail:

"The mail message frame is not fixed. If you read a new message, and scroll down to the bottom and decide you don't want to keep it, you can no longer access the delete button. Yes, obviously you can just close the email and then choose it again and delete, but that is a total waste of time and key strokes."

"Yahoo disappears emails sent between legitimate and active Yahoo accounts."

"I used Yahoo mail as a central box for most of my domain emails. After switching a couple of weeks ago and sending 30 test mails over three days to make sure it was all working, 6 never arrived. I even have specific filters set-up within Yahoo mail to make sure certain mails do not get put in the trash or spam by mistake."

"It's very laggy, and that pause of a second or two between commands makes it extremely frustrating."

"The new version of Yahoo mail makes all my messages older than a week or so unreadable. I get "Error 45" and the messages fail to load. There are numerous reports about this on the net; so far Yahoo hasn't managed to fix it. So, I have to launch the classic version of Yahoo mail just to read old messages. Fail."

"Right after the switch I was hacked and my contacts all 500 plus were gone without a trace."

Besides the speed with which it was built, another reason that Yahoo Mail may be broken is that Shashi Seth, who led its development, no longer works at the company. Mayer asked him to leave Yahoo in January.

The only good news for Mayer in all this is that Yahoo Mail usage hasn't collapsed, despite all the complaining.